The present invention relates to automatically adjusting the relative positioning of two printer drums based on a determined level of contact between the drums and, more particularly, but not exclusively to adjusting the positioning of drums of an electrophotographic printer.
Many forms of printing rely on a printing drum whose rotation transfers an image from the drum to a printed substrate. More advanced forms of printing, such as electrophotographic printing, utilize parallel pairs of drums whose joint rotation transfers the image either from one drum to the next, or from one drum to a substrate supported by another drum. Electrophotographic printing machines generally use a two-transfer system of printing in which an electrophotographic image is formed on a first drum (known as the PIP drum) using a laser beam shone onto a photoelectric material, thereby forming an electrostatic image on the photoelectric material. Ink is then drawn into the electrostatic image. The image so formed is then transferred in a first transfer operation onto a blanket carried by an intermediate transfer drum, known as the ITM drum. A second transfer operation occurs when the image is transferred from the blanket onto the printing substrate which is held on a third drum, known as the impression drum.
Referring now to the drawings, FIG. 1 schematically illustrates a cross sectional view of an electrostatic printing assembly 1, according to the teaching of prior art. Apparatus 1 comprises an electrostatic drum 10 (also denoted herein the PIP drum) arranged for rotation about an axle 12. Drum 10 is typically formed with an imaging surface 16, e.g., a photoconductive surface. Surface 16 is typically of a cylindrical shape.
A charging unit 18, which can be a corotron, a scorotron, a roller charger or any other suitable charging unit known in the art, uniformly charges surface 16, for example, with positive charge.
Continued rotation of the drum 10 brings surface 16 into image receiving relationship with an exposing unit 20, which focuses one or more scanning laser beams onto surface 16 to scan a desired image. The laser beams selectively discharge surface 16 in the areas struck by light, thereby forming an electrostatic latent image. Usually, the desired image is discharged by the light while the background areas are left electrostatically charged. Thus, the latent image normally includes image areas at a first electrical potential and background areas at another electrical potential. Unit 20 may be a modulated laser beam scanning device, an optical focusing device or any other imaging device known in the art.
Continued rotation of the drum 10 brings imaging surface 16, now bearing the electrostatic latent image, into a developing unit 22, which typically comprises electrodes 24 that apply a liquid toner or ink on surface 16, so as to develop the electrostatic latent image. The liquid toner can comprise charged solid particulates dispersed in a carrier liquid. The solid particulates are typically charged to the same polarity as the photoconductor. Thus, due to electrostatic repulsion forces, ink particles adhere to areas on the photoconductor corresponding to the image regions, substantially without adhering to, and thus developing, the background regions. In this manner a developed image is formed on surface 16.
Following application of liquid toner thereto, surface 16 typically passes through other rollers (not shown) which ensure that the ink surface is appropriate for transfer to ITM drum 40. A first ink transfer then occurs, in which the liquid image is transferred, typically via electrostatic attraction, from drum 10 to ITM drum 40, rotating in the opposite direction of drum 10. In order for the first transfer to occur, an electrical bias is needed in the direction of image transfer. The drums are therefore generally biased by a bias unit, so that a forward bias leads from electrostatic drum 10 to ITM drum 40.
Subsequently, the image experiences a second transfer, typically aided by heat and pressure, from ITM drum 40 to a substrate 42, which is supported by an impression drum 43.
Following the transfer of the liquid image to ITM drum 40, imaging surface 16 is cleaned to remove ink traces. Residual charge left on surface 16 can be removed, e.g., by flooding surface 16 with light from a lamp 58.
In electrophotographic printing, print quality and overall machine performance are both highly dependent on the pressure between the drums and on the parallel alignment of the drums. This problem also appears in conventional printing presses, and other equipment that need smooth rotation of two cylinders with precisely controlled gap. The first transfer pressure (i.e. between the PIP and the ITM drums) contributes to several print quality parameters including:
(a) Small (single and double pixel) dots transfer
(b) Solid quality (fog and small voids in solid ink layer)
(c) Quality of horizontal lines
(d) Short Term Memory (STM) and wetness
(e) Banding, especially on gray portions of the image and horizontal lines
(f) Background transfer
An improper first transfer pressure can also degrade overall machine performance parameters by:
(a) Decreasing the blanket lifetime (related to background transfer)
(b) Increasing the amount of ink amount per area unit (dma) and, as a result, ink consumption, reservoir filter lifetime, and fixing
(c) Decreasing the PIP life span
Similarly, an incorrect second transfer pressure (between the ITM drum surface and the blanket on the impression drum) can also decrease blanket life span as well as increase the possibility of paper jams. The printing blanket and paper thickness both contribute to the second transfer pressure, so that pressure changes may be caused by inconsistent printing blanket thickness.
One method of reducing banding on print is to use bearers to fix the gap between two drums. Bearers are rigid shoulders on each cylinder with a diameter slightly larger than the center of cylinder. However, the bearers create a fixed gap which cannot compensate for changes in blanket thickness or other changes. Therefore the pressure between the plate and blanket may not be optimal, leading to deficiencies in image quality of the print.
Currently the inter-drum pressure is commonly adjusted manually. Typically, images are first printed at different pressures. An operator then visually inspects the resulting printed pages, and selects the correct pressure on the basis of the visual analysis. For example, the operator can insert an under-packing material below the blanket and\or plate to compensate for such changes. This requires high-skilled manual operation, extra materials, and is hard to implement. An alternate solution is to use conical shaped bearers and to control the axial alignment of the cylinders. However, designing a drum with conical bearers is complicated and increases the hardware costs of the press. Additionally, the axial movement requires printer elements to be a little wider in order to compensate for the varying printing width. In another solution, the PIP drum position is adjusted by running motors, based on the type of print medium selected by the operator. This method requires operator input and is therefore prone to human error. Furthermore, positioning the print drum for a particular print medium does not account for other factors, such as material tolerances, temperature variations, and so forth.
In summary, the current methods for ensuring correct transfer pressures suffer from several disadvantages. The process is operator dependent, yielding a difference in pressure between customers, and in many cases is not even performed. Even when performed, the adjustment process is generally not of high enough precision, so that the best possible performance is not always obtained. Additionally, since the pressure changes during printing, adjusting the pressure before printing does not ensure that the pressure is optimal during printing.
There is thus a widely recognized need for, and it would be highly advantageous to have, an apparatus and method for controlling the pressure between drums devoid of the above limitations.